1. Field of the Invention
The present invention relates to apparatus, such as shock absorbers, for damping vibration in a motor vehicle, and more particularly to such apparatus having a dynamically adjustable damping characteristic.
2. Description of the Related Art
Hydraulic shock absorbers are commonly placed between the axles and the frame of a motor vehicle to reduce transmission of vibration from the wheels. Large trucks and off-road vehicles used in construction and agriculture incorporate similar vibration damping devices between the vehicle frame and the operator cab or between a seat and the vehicle body. The purpose of all these apparatus is to isolate the occupants from vibrations produced as the vehicle travels over the ground.
A typical prior hydraulic vibration damper comprised a cylinder divided by a damping piston into two working chambers filled with a fluid. The cylinder was attached to either the axle or the frame of the vehicle and the piston was attached by a rod to the other vehicle component. Thus movement of the axle relative to the frame caused the piston to slide within the cylinder thereby expanding one chamber and contracting the other chamber. Motion which extends the piston rod from the cylinder is referred to as rebound and motion in the opposite direction is compression. The damping piston had one or more fixed orifices through which the fluid flowed between the cylinder chambers. The orifices restricted the flow rate of that fluid flow thereby limiting the rate of piston movement to dampen the vibration. Such prior apparatus provided a fixed damping force for any given velocity.
Subsequently, adjustable vibration dampers were developed that included a bypass passage arranged between the two working chambers. An electrically operated proportional valve and a pressure-dependent valve were placed in series and activated during rebound and compression. The activation of the electrically operated valve was controlled in response to vibration of the vehicle detected by a sensor and opened the bypass passage by an amount that provided proportionally variable damping effect. The pressure-dependent valve opened only in response to pressure exceeding a defined level.
The bypass passage and its valves were mounted outside the vibration damper cylinder and increased the space required for that assembly. It is desirable to incorporate the functionality of the bypass passage into the cylinder and make a more compact assembly.